/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int sumNumbers(TreeNode* root) {
        return dfs(root, 0);
    }

    int dfs(TreeNode* root, int prevsum)
    {
        prevsum = prevsum * 10 + root->val;

        if (root->left == nullptr && root->right == nullptr)
            return prevsum;

        int ret = 0;
        ret += root->left == nullptr ? 0 : dfs(root->left, prevsum);
        ret += root->right == nullptr ? 0 : dfs(root->right, prevsum);

        return ret;
    }
};